【问题描述】[中等]

【解答思路】

1. 减法

import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Deque;
import java.util.List;public class Solution {public List<List<Integer>> combinationSum2(int[] candidates, int target) {int len = candidates.length;List<List<Integer>> res = new ArrayList<>();if (len == 0) {return res;}// 关键步骤Arrays.sort(candidates);Deque<Integer> path = new ArrayDeque<>(len);dfs(candidates, len, 0, target, path, res);return res;}/*** @param candidates 候选数组* @param len        冗余变量* @param begin      从候选数组的 begin 位置开始搜索* @param target     表示剩余，这个值一开始等于 target，基于题目中说明的"所有数字（包括目标数）都是正整数"这个条件* @param path       从根结点到叶子结点的路径* @param res*/private void dfs(int[] candidates, int len, int begin, int target, Deque<Integer> path, List<List<Integer>> res) {if (target == 0) {res.add(new ArrayList<>(path));return;}for (int i = begin; i < len; i++) {// 大剪枝if (target - candidates[i] < 0) {break;}// 小剪枝if (i > begin && candidates[i] == candidates[i - 1]) {continue;}path.addLast(candidates[i]);// 因为元素不可以重复使用，这里递归传递下去的是 i + 1 而不是 idfs(candidates, len, i + 1, target - candidates[i], path, res);path.removeLast();}}public static void main(String[] args) {int[] candidates = new int[]{10, 1, 2, 7, 6, 1, 5};int target = 8;Solution solution = new Solution();List<List<Integer>> res = solution.combinationSum2(candidates, target);System.out.println("输出 => " + res);}
}

2. 加法

class Solution {public List<List<Integer>> res = new ArrayList<>();public List<List<Integer>> combinationSum2(int[] candidates, int target) {int len = candidates.length;boolean[] used = new boolean[len];Arrays.sort(candidates);dfs(candidates,target,new ArrayDeque<Integer>(),0,0);return res;}public void dfs(int[] cand,int target,Deque<Integer> temp,int sum,int index){if(sum>target) return;if(sum==target){res.add(new ArrayList<>(temp));return;}for(int i=index;i<cand.length;++i){if (i > index && cand[i] == cand[i - 1]) {continue;}temp.addLast(cand[i]);dfs(cand,target,temp,sum+cand[i],i+1);temp.removeLast();}}
}

【总结】

1. 剪枝说明

2.回溯算法相关题目

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转载链接：https://leetcode-cn.com/problems/combination-sum-ii/solution/hui-su-suan-fa-jian-zhi-python-dai-ma-java-dai-m-3/